Hi SJH, nope, re post #91, I assumed the rails were already set-up and so could be used for a level surface and checking point.

What I meant was the headstock bottom must first be made level and true to the spindle centre line by using a test bar in the chuck and dialing it on the surface plate, after which it can be mounted on the prepared base and checked for allignment to the rails.

You're quite right about the above/below centre differences not making a 'haporth worth of difference, as long as you get the 150mm (6") out good to a thou' the rest will just go along for the ride.

Incidently, and just as a matter of interest, a turret lathe has the centre height very accurately alligned because they use a "knee tool" to produce parallel work on a repetition basis, and this tool cuts along the top of the work from the turret slide, and it is this function that allows work to be held to diam tolerances when the operator is pushing the production, but for your set-up the height allignment is not that critical, unless you are going to have tools in the turret that cut on the top of the bar.

I read back over the posts to get the background again, one of your best decisions was to get the surface plate.

I think if you had opened a thread with the proposal to build this lathe from a design concept without drawings, you would have got a million reasons why it could not be done without a whole heap of expensive gear, but it just goes to show you, if you get the show on the road and work round the design it will eventually fall into place.
Congratulations, can't wait to see the wheels go round.
Ian.